The present invention relates generally to positive displacement piston pumps and, more particularly, is directed to a submersible positive displacement piston pump having particular applicability to the delivery of small quantities of liquid to an application device.
For metering small amounts of liquid from a transport drum to an application device, it is well known to use pumps. For example, U.S. Patent No. 4,583,920, and having a common assignee herewith, describes an air driven diaphragm pump which discharges liquid pulses at a constant rate and which is used to transfer a few liters of chemicals per day from a transport drum to an application point. The application point, as an example, can be a coating hood for coating glass bottles.
An advantage of this pump is that it uses air pressure operated valves which prevent liquid leakage commonly found in pumps using conventional ball check valves. However, while this pump produces good results, differences in stroke volume of, for example, 20%, due to workmanship and membrane quality, may exist between different air driven diaphragm pumps.
In order to overcome these deficiencies, positive displacement piston pumps, such as disclosed in U.S. Patent Nos. 3,168,872; 3,257,953; and 4,008,003, and particularly, as modified in U.S. Patent No. 4,536,140 having a common assignee herewith, have been used. The entire disclosure of U.S. Patent No. 4,536,140 is incorporated herein by reference. The pump in the latter U.S. Patent has been found suitable for pumping liquid organometallic compounds used in the coating of glass bottles with tin oxide or other metal oxides.
With such pumps, the pumping chamber is kept out of th transport drum and a hose from the fluid inlet extends into the drum. Thus, the chemicals are pumped out of the drum, through the hose, and into the pumping chamber. However, as discussed in U.S. Patent No. 4,536,140, a problem with positive displacement piston pumps is that the chemical may migrate from the pumping chamber to a space between the piston and cylinder walls, even with the closet of tolerances between the reciprocating piston and the interior cylinder wall. Where the chemical is a corrosive material such as a monobutyltin trichloride containing formulation, the minute migration of liquid chemical may result in the build-up of metalhydroxy compounds between the reciprocating piston and inner cylinder wall. Such compounds are formed by reaction of the chemical with water vapor in ambient air. Because of such build-up, the piston may freeze in the cylinder, causing failure of the apparatus.
The aforementioned U.S. Patent No. 4,536,140 attempts to overcome these deficiencies by providing a separate oil barrier with constant oil pressure between the piston and inner cylinder wall. However, the use of such a barrier greatly complicates the apparatus. Further, the sealing oil, dependent upon the application, may interfere with the pump operation.
Another problem with such pumps is that minute amounts of air entering the pump may interfere with proper operation, resulting in a lower pumping rate. Specifically, gases, such as air, hydrogen, carbon dioxide and the like which are carried in the fluid, are often released in the cylinder as a result of agitation of the fluid during the pumping operation or as a result of pressure and temperature changes. For example, some fluids respond to agitation and/or pressure and temperature changes by chemically separating into liquid and gas fractions, while other fluids simply vaporize, physically changing from liquid to gaseous form. The problem that results is that the gases form bubbles which become trapped in the pumping head of the cylinder, thereby spoiling the metering precision of the pump, and in some situations, blocking flow completely. Generally, the gas bubbles become trapped between the recessed section of the piston and the inner wall of the cylinder.
Specifically, when the pump is not operating at full capacity, that is, when the piston is pivoted to less than its maximum extent, the piston is caused to reciprocate over a lesser distance between its retracted position and extended position. As a result, the top of the recessed section remains above the outlet port at all times during reciprocation of the piston. Gas bubbles formed between the recessed section and the inner wall of the cylinder thereby remain during the pumping operation, adversely affecting the same. It will be appreciated that the smaller the piston stroke, the more gas that will be trapped by the recessed section, thereby increasing the ratio of volume of entrapped gas to pump displacement. In other words, the pump becomes gas sensitive.
Because of this problem, a pump operating at less than maximum capacity must have its flow rate changed several times. Entrapped gas will then flow out of the pump, restoring its set delivery rate. However, such capacity changes are bothersome and time consuming. When used, for example, for pumping fluid to coat bottles, such capacity changes cause excess usage of expensive coating chemicals or cause insufficient coating on the bottles.
This latter problem of minute amounts of air entering the pump has been solved by the invention of U.S. Patent 4,575,317, the entire disclosure of which is incorporated herein by reference.